The human anorectal sphincter is a composite structure consisting of unique smooth and striate muscle, and having several anatomic configurations that are innervated from different sources. Both relaxation and contraction of the sphincter can occur simultaneously and at different loci within the sphincter. An adequate understanding of what is occurring at any single point, or at any single cross-sectional plane, is not always sufficient to understand the operation of the entire sphincter. Current recording mechanisms do not permit simultaneous assessment of discrete loci throughout the entire sphincter whether under voluntary control or in response to stimuli.
Numerous techniques have been used to assess various aspects of anorectal function. The techniques can generally be grouped into four categories: radiographic defecography, electromyography, rectal compliance, and anorectal sphincter manometry. Anorectal sphincter manometry, or perhaps more accurately "myography", typically uses measurements of fluid pressure to provide an objective assessment of various aspects of anorectal sphincter function. It provides a far more reliable indicator of the anal sphincter tone than can be achieved by digital examination. The relative contribution of the voluntary and involuntary components can be assessed, and the integrity of reflex inhibition to rectal distention can be evaluated. Moreover, the presence of an abnormal manometric pressure profile can provide insight regarding symptoms, etiology, and treatment.
In the most common sphincter manometry systems, such as a perfusion probe, fluid, typically in the form of an aqueous solution, is perfused at a constant rate through one or more small side ports in the probe. A transducer measures pressure to determine the resistance that the sphincter presents to perfused fluid. Another version of a perfusion probe is an infusion catheter. The catheter includes several holes near its end and radially arranged so that fluid can be pumped into the catheter and radially out through the holes. Measuring the back pressure on the perfused fluid provides a measure of the radially inward pressure exerted by the musculature of the sphincter on the catheter at the multiple locations of the holes. The holes are all circumferentially arranged around the catheter at a single axial position or in a close spiral pattern along the catheter. As a result, it is believed that the greatest number of holes used to date is eight. The close pattern of the holes results in only one position along the axis of the; sphincter being measured at any one time. Consequently, it is necessary to pull the catheter at a constant rate so that the perfusion holes travel along the entire length of the sphincter. Measurements are taken periodically as the catheter moves to provide a pressure profile of the entire sphincter. Good resolution is achieved when there are eight radially directed holes, circumferentially spaced around the catheter so that eight measurements can be taken simultaneously at each axial location.
Taking a sequence of pressure measurements at corresponding axial positions of the perfusion catheter over the inner surface of the sphincter has proven to be of assistance in assessment of a normal sphincter, and is a distinct improvement over prior less sophisticated methods. Typically, as the catheter moves, readings are taken every one-half millimeter, and consequently, assessing an entire sphincter can require some 2000 pressure readings and can take one hundred seconds. The readings are usually performed according to a protocol where, for example, the patient is required to relax and contract the sphincter, thereby squeezing the catheter. Data for each reading is stored in a separate file. Files are stored and various measurements are determined from the data, such as average pressure and maximum pressure.
A profile can also be drawn of the sphincter at each axial location that a reading is taken. Present systems use a chart recorder to record a sequence of readings. However, because the measurements for each reading are typically taken over one hundred seconds, noise and various artifacts are introduced. In addition, the amount of information that can be acquired is limited due to mechanical and physical limitations. For example, the number of perfusion channels that can be used is limited because as the number of channels increases, the amount of water required increases too, potentially resulting in patient discomfort and recording artifact. An alternative approach is to use multiple catheters, but this approach creates mechanical problems, since the device, is too large, and again, too much perfusion fluid is required. In addition, as the catheter is pulled, static and dynamic friction may result in incorrect readings. Also, during contraction maneuvers the patient can fatigue over the 100 seconds required to scan over an entire sphincter, resulting in some relaxation of the sphincter before the scan is complete. Finally, these devices tend to be very expensive.
Simultaneous multipoint recording has contributed substantially to the development of a more comprehensive picture of sphincter characteristics and action.
In addition to the perfusion probe, several other instruments have been developed for measuring a constriction pressure profile of a sphincter, including the anal, urethral, and esophageal sphincters. One type of probe employs a nueroballoon. Another approach uses pressure microtransducers on a catheter instead of perfusion holes. Nevertheless, the catheter must be pulled through the sphincter and readings sequentially taken to provide a complete assessment.
Further microtransducers, while being solid state, unlike the perfusion probes, are typically very expensive. In addition, a catheter based on known solid state microtransducers would be very large, and a catheter having a radial arrangement of known microtransducers at a single axial location on the catheter would still have to be pulled through a sphincter to acquire a complete pressure profile of the sphincter.